Add 'messages' logger for cleaner logging of human-readable messages

The archetype project logs several messages that are meant for human consumption. Examples are the messages from auto-export and the updater, but also the auto-import of watched web resources should log some messages.

The layout of the default console appender is too verbose for these kind of messages. A developer is not interested in the originating class, line number, etc, just in the message itself. There should be a separate logger for these kind of messages.